The present invention relates to apparatus and methods for measuring stress in thin films (membranes) in specimens, particularly specimens such as reticles and reticle blanks as used in charged-particle-beam microlithography apparatus and methods.
In recent years, as individual circuit elements in semiconductor integrated circuits have become increasingly miniaturized, microlithographic exposure systems and methods offering prospects of finer resolution than obtainable by optical microlithography (which is encumbered by the diffraction limit of light) have received great RandD attention. For example, microlithographic technologies utilizing a charged particle beam (e.g., ion beam or electron beam) or X-ray beam are currently under intensive development.
One conventional microlithographic system utilizes a single electron beam to xe2x80x9cdrawxe2x80x9d the pattern on a substrate (e.g., semiconductor wafer) line-by-line. With such technology, a fine pattern having a linewidth of 1xcexcm or less can be produced since the electron beam can be constricted to a diameter of a few xc3x85ngstroms. Unfortunately, progressive miniaturization of a circuit typically results in a more condensed circuit with a correspondingly greater number of lines. Because these systems xe2x80x9cdrawxe2x80x9d the pattern line-by-line, a more condensed circuit has more lines and requires a more constricted beam. Consequently, drawing time is increased, and xe2x80x9cthroughputxe2x80x9d (number of circuits or wafers that can be processed per unit time) is correspondingly decreased. Low throughput is a key factor in making this technique unsuitable for mass-production of integrated circuits.
Another approach receiving substantial current attention is charged-particle-beam (CPB) projection microlithography in which a pattern defined on a reticle or mask is illuminated and projected (xe2x80x9ctransferredxe2x80x9d), using the charged particle beam, to the substrate. Referring to FIG. 3(a), one type of reticle used with CPB projection microlithography is a scattering-membrane reticle 21 comprising a membrane 22 on which a pattern of scattering bodies 24 is formed. The scattering bodies 24 scatter particles of an incident charged particle beam, while the membrane 22 is relatively transmissive to such particles without scattering the beam. Referring to FIG. 3(b), another type of reticle used with CPB projection microlithography is a scattering-stencil reticle 31 comprising a membrane 32 defining a pattern of voids (through-holes). The membrane 32 has a thickness sufficient to scatter particles of an incident charged particle beam, whereas the particles pass freely through the voids.
In each of the reticles shown in FIGS. 3(a)-3(b), the pattern is divided into multiple small regions 22a, 32a each defining a respective portion of the overall pattern defined by the respective reticle. The small regions 22a, 32a are separated from each other by respective boundary regions 25, 35 that define no portion of the pattern. The boundary regions typically include struts 23, 33 extending therefrom. The struts add substantial rigidity and mechanical strength to the reticle.
Conventionally, reticles such as those shown in FIGS. 3(a)-3(b) are fabricated from xe2x80x9creticle blanks,xe2x80x9d which are essentially reticles lacking any pattern. Reticle blanks used for making scattering-stencil masks can be manufactured using, for example, a manufacturing process described in Japanese Kxc3x4kai Patent Publication No. Hei 10-106943. In this process, fabrication of a reticle blank begins with formation of an SOI (silicon-on-insulator) substrate. The SOI substrate is prepared from a supporting silicon substrate, a silicon oxide layer on the silicon substrate, and a silicon xe2x80x9cactivexe2x80x9d (doped) layer on the silicon oxide layer. The silicon substrate is etched to form the struts, with regions of membrane extending between the struts. In forming the SOI substrate, the silicon oxide layer is formed by thermal oxidation of the respective surface of the silicon substrate. This oxidation step, and a subsequent step in which the silicon active layer is fused thermally with the silicon oxide layer, involve heating to respective temperatures greater than 1000 degrees. Unfortunately, due to differences in the thermal-expansion coefficients of silicon oxide versus the silicon substrate, residual thermal compressive and tensile stresses are generated in the silicon active layer as the reticle blank returns to a normal temperature. These residual stresses cause deformation of the membrane. If the residual stresses are excessively large, then substantial deformation of the pattern can arise when the reticle blank is made into a reticle.
Accordingly, to form a reticle in which the pattern formed on the reticle membrane has an optimal level of internal stress, it is highly desirable to measure and evaluate the residual internal stress of the membrane. An important known technique for obtaining such measurements and evaluations of residual membrane stress is discussed in Anderer and Behringer, xe2x80x9cDetermination of the Average Stress and Its Adjustment in Thin Silicon Membranes Used in Various Lithographies,xe2x80x9d Microelectronic Engineering 5:67-71,1986.
A conventional stress-measuring apparatus is shown in FIG. 4. The FIG. 4 apparatus includes a stage 42 on which the specimen (i.e., reticle or reticle blank) is mounted, a vibration-applying electrode 43 used to apply a vibration to a membrane 41a of the specimen 41, and a detection electrode 44 situated on the opposite side of the specimen 41 from the electrode 43. An AC voltage (at a desired selectable frequency) is supplied to the electrode 43 to cause the electrode 43 to generate an AC electrical field. The vibration-applying electrode 43 and detection electrode 44 normally are situated in extremely close proximity to respective surfaces of the membrane 41a. The AC electrical field electrostatically extends from the electrode 43 to the membrane 41a. Hence, the vibration-applying electrode 43 applies a vibration (corresponding to the frequency of the AC electrical field) electrostatically to the membrane 41a. As the membrane 41a is being energized in this manner, a resonance frequency of vibration of the membrane 41a is detected as corresponding changes in capacitance between the membrane 41a and the detection electrode 44.
A stress-frequency table is prepared for respective calibration membranes as determined by finite-element analysis from data concerning the length, thickness, density, Poisson ratio, and Young""s modulus values of calibration membranes. The table is stored in a memory. A stress value corresponding to a particular measured resonance frequency is determined from the data in the table.
As noted above, the vibration-applying electrode 43 must be situated in extremely close proximity to the membrane 41a of the specimen 41, i.e., at a distance of 300 xcexcm or less. The detection electrode 44 also must be placed in similar close proximity to the membrane 41a of the specimen 41. Unfortunately, because accurate placements of the electrodes 43, 44 relative to the membrane 41a require significant time to perform, measurement throughput is low.
Furthermore, because the conventional stress-measuring device summarized above utilizes a vibration-applying electrode 43 and detection electrode 44, the device only can be used to measure stress in a conductive membrane.
In view of the shortcomings of the prior art as summarized above, an object of the invention is to provide stress-measurement apparatus and methods exhibiting improved measurement throughput. Another object is to provide such apparatus and methods that can be used to measure stress in any desired membrane, including non-conductive membranes.
According to a first aspect of the invention, apparatus are provided for measuring stress in a membrane of a specimen. A representative embodiment of such an apparatus comprises a vibration-applying device situated and configured to apply a vibration stimulus to the membrane from a distance (desirably in space from the vibration-applying device to the membrane), for example, 1.5 mm or greater. The vibration stimulus causes the membrane to experience a vibrational stress. The apparatus also comprises a detector situated and configured to measure the stress.
Desirably, the detector comprises a light source situated and configured to irradiate a region of the membrane with light such that at least a portion of the light is reflected from the membrane. The detector desirably also comprises a light receiver situated and configured to receive the light reflected from the irradiated region of the membrane and produce a corresponding output signal. Further desirably, the light receiver is connected to a computer configured to determine, from the output signal, a vibration spectrum for the irradiated region and to determine, from the vibration spectrum, a measure of a stress characteristic of the region of the membrane.
The apparatus can further comprise a signal processor connecting the light receiver to the computer. The signal processor is configured to receive the output signal from the light receiver and to convert the output signal to a corresponding digital signal routed to the computer.
The vibration-applying device desirably is configured as a piezo-electric element, and the light source desirably is a laser.
According to another aspect of the invention, methods are provided for measuring stress in a membrane of a specimen. In a representative embodiment of such a method, a vibration stimulus is directed from a source over a distance to a region of the membrane. The vibration stimulus causes the region of the membrane to vibrate. As the region of the membrane is being caused to vibrate, the region is irradiated with a light so as to produce light reflected from the region of the membrane. The reflected light is received and detected to produce a corresponding detection signal. The detection signal is converted into a corresponding vibration spectrum indicating variations in membrane-vibration amplitude with respect to vibration frequency. From the vibration spectrum, at least one resonance frequency of the membrane is determined. From the resonance frequency, a value of stress of the membrane is determined. The determination of stress desirably is performed by comparing data concerning the resonance frequency with a table of data concerning stress versus frequency for one or more membranes. The table can be stored in a memory and recalled for performing this determination. The table can be prepared by performing a finite-element analysis from data on length, thickness, density, Poisson ratio, and Young""s modulus of various membranes.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.